A magnetic recording read channel converts an analog read channel into an estimate of the user data recorded on a magnetic medium. Read heads and magnetic media introduce noise and other distortions into the read signal. As the information densities in magnetic recording increase, the intersymbol interference (ISI) becomes more severe as well. In read channel chips, a Viterbi detector is typically used to detect the read data bits in the presence of intersymbol interference and noise.
The Soft Output Viterbi Algorithm (SOVA) is a well known technique for generating soft decisions inside a Viterbi detector. A soft decision provides a detected bit with a corresponding reliability. These soft decisions can be used by an outer detector to improve the error rate performance of the overall system. For a more detailed discussion of SOVA detectors, see, for example, J. Hagenauer and P. Hoeher, “A Viterbi Algorithm with Soft-decision Outputs and its Applications,” IEEE Global Telecommunications Conference (GLOBECOM), vol. 3, 1680-1686 (November 1989).
Various parameters of a magnetic recording read channel are typically adjusted to improve the Bit Error Rate (BER) performance. While measuring the BER provides the most accurate measure of performance, the BER measurement is unduly time consuming. A number of techniques have been proposed or suggested for obtaining performance measures based on the mean squared error or other derivatives of an error term derived from decoding the bit sequence and then re-creating the ideal pattern. Although these techniques are generally fast, they are not guaranteed to correlate directly with the BER.
A need therefore exists for an improved method and apparatus for obtaining performance measures without measuring the BER. A further need exists for a method and apparatus for obtaining performance measures in a read channel that are directly correlated to BER but takes much less time to collect.